The ephrin receptors (Eph receptors) are a large family of receptor tyrosine kinases that regulate a multitude of processes in developing and adult tissues by binding a family of ligands called ephrins. Eph receptors are divided into either the A- or B-type with ephrin ligands. There are currently nine known members of the A-type, EphA1-8 and EphA10, and four known members of the B-type, EphB1-4 and EphB6. In general, the A class receptors preferentially bind A-type ligands, while the B class receptors preferentially bind the B-type ligands. The Eph receptors are like other receptor tyrosine kinases, with a single transmembrane spanning domain, with a glycosylated extracellular region comprised of a ligand-binding domain with immunoglobulin-like motifs, a cysteine rich region and two fibronectin type III repeats. (Surawska et al., Cytokine & Growth Factor Reviews 15:419-433 (2004)). The ephrin ligands are divided into the A and B class depending on their sequence conservation. EphrinA ligands are glycosylphosphatidylinisotol anchored and usually bound by Eph-A type receptors, while ephrinB ligands contain a transmembrane domain and a short cytoplasmic region and are usually bound by EphB-type receptors. Id.
The signaling process begins when Eph receptor dimerizes with an ephrin ligand, causing the receptor to become phosphorylated. Aggregates of ephrin-EphReceptor complexes are formed by higher-order clustering. Receptor activation is thought to depend on the degree of multimerization, but is not limited to the tetrameric form as receptor phosphorylation is observed in both lower- and higher-order forms. Depending on the state of multimerization, distinct Eph receptor complexes can induce biological effects. In addition to the “forward” signaling through the Eph receptor into the receptor-expressing cell, there is also “backwards” signaling through the ephrin into the ephrin-expressing cell. For example, the cytoplasmic tail on the B-ephrins can become phosphorylated leading to the recruitment of signaling effectors and a signal transduction cascade within the ephrin-signaling cell. Id.
Ephrins are now known to have roles in many cell-cell interactions, including axon pathfinding, neuronal cell migration, and interactions in vascular endothelial cells and specialized epithelia. (Flanagan & Vanderhaeghen, Annu. Rev. Neurosci. 21:309-345 (1998); Frisen et al., EMBO J. 18:5159-5165 (1999)). Eph receptors have also been implicated in a variety of pathological processes, including tumor progression, pathological forms of angiogenesis, chronic pain following tissue damage, inhibition of nerve regeneration after spinal cord injury, and human congenital malformations. (Koolpe et al., J Biol. Chem. 280:17301-17311 (2005)). Eph receptors are also reported to play a role in the balance of stem cell self-renewal versus cell-fate determination and differentiation. Id.
Eph receptor and ephrin over-expression can result in tumorigenesis, and are associated with angiogenesis and metastasis in many types of human cancer, including lung, breast and prostate cancer, as well as melanoma and leukemia. (Surawska et al., Cytokine & Growth Factor Reviews 15:419-433 (2004)). Over-expression of the Eph receptor is thought not to affect the proliferation of cells, but changes their invasive behavior. According to one theory, in malignant cells with high levels of EphA2, the receptors are mislocalized, not able to bind their ephrin ligands, and therefore not phosphorylated, resulting in increased extracellular matrix adhesions and higher metastatic potential. (Ruoslahti, Adv. Cancer Res. 76:1-20 (1999)). Angiogenesis is the formation of new blood vessels and capillaries from pre-existing vasculature and is an essential process for tumor survival and growth. Evidence exists that implicates Eph receptor/ephrin up-regulation during blood vessel invasion of tumors. (Surawska et al., (2004).) A-type ephrins in particular are associated with tumor angiogenesis, and EphA2-Fc and EphA3-Fc fusion proteins decreased tumor vascular density, tumor volume and cell proliferation, and also increased apoptosis. (Brantley et al., Oncogene 21:7011-7026 (2002)).
The crystal structure of the EphB2 receptor-ephrinB2 complex indicates that the ectodomain of the ephrinB2 folding topology is an eight-stranded barrel that is a variation on the common Greek key P-barrel fold, and shares considerable homology with the cupredoxin family of copper-binding proteins, although ephrinB2 does not bind copper. The main difference between ephrin and the cupredoxin-fold proteins is the unusual length of the ephrin G-HL and C-DL loops with are part of the dimerization and tetramerization ligand receptor interfaces, respectively. Crystallization studies further indicate that the G-HL loop is involved in receptor binding. (Himanen et al., Nature 414:933-938 (2001)). The extracellular domain of mouse ephrinB2 also has a topological similarity to plant nodulins and phytocyanins. (Toth et al., Developmental Cell, 1:83-92 (2001)).
Reports on regression of cancer in humans and animals infected with microbial pathogens date back more than 100 years, originating with the initial report by Coley. (Clin. Orthop. Relat. Res. 262:3-12 (1891)). Several subsequent reports have shown that microbial pathogens replicate at tumor sites under hypoxic conditions and also stimulate the host's immune system during infection, leading to an inhibition of cancer progression. (Alexandrof et al., Lancet 353:1689-1694 (1999); Paglia & Guzman, Cancer Immunol. Immunother. 46:88-92 (1998); Pawelek et al., Cancer Res. 57:4537-4544 (1997)). Bacterial pathogens such as Pseudomonas aeruginosa and many others produce a range of virulence factors that allow the bacteria to escape host defense and cause disease. (Tang et al., Infect. Immun. 64:37-43 (1996); Clark and Bavoil, Methods in Enzymology, vol. 235, Bacterial Pathogenesis, Academic Press, Inc. San Diego Calif. (1994); Salyers and Whitt, Bacterial Pathogenesis: A Molecular Approach, ASM Press, Washington D.C. (1994)). Some virulence factors induce apoptosis in phagocytic cells such as macrophages to subvert host defense. (Monack et al., Proc. Natl. Acad. Sci. USA 94:10385-10390 (1997); Zychlinsky and Sansonetti, J. Clin. Investig. 100:493-495 (1997)).
Two redox proteins elaborated by P. aeruginosa, the cupredoxin azurin and cytochrome c551 (Cyt c551), both enter J774 cells and show significant cytotoxic activity towards the human cancer cells as compared to normal cells. (Zaborina et al., Microbiology 146: 2521-2530 (2000)). Azurin can also enter human melanoma UISO-Mel-2 or human breast cancer MCF-7 cells. (Yamada et al., PNAS 99:14098-14103 (2002); Punj et al., Oncogene 23:2367-2378 (2004); Yamada et al., Cell. Biol. 7:14181431 (2005)). In addition, azurin from P. aeruginosa preferentially enters J774 murine reticulum cell sarcoma cells, forms a complex with and stabilizes the tumor suppressor protein p53, enhances the intracellular concentration of p53, and induces apoptosis. (Yamada et al., Infection and Immunity, 70:7054-7062 (2002)). Azurin also caused a significant increase of apoptosis in human osteosarcoma cells as compared to non-cancerous cells. (Ye et al., Ai Zheng 24:298-304 (2003)).
Cytochrome C551 (Cyt C551) from P. aeruginosa enhances the level of tumor suppressor protein p16Ink4a and inhibits cell cycle progression in J774 cells. (Hiraoka et al., PNAS 101:6427-6432 (2004)). However, when colon cancer cells, such as HCT 116 cells, or p53-null lung cancer H1299 cells were grown in presence of wild type azurin or wild type cytochrome c551 for 3 days, they inhibited the growth of HCT 116 cells at a much lower concentration (IC50=17 μg/ml for azurin; 12 μg/ml for Cyt C) than H1299 cells (>20 μg/ml). Id.
A cancer is a malignant tumor of potentially unlimited growth. It is primarily the pathogenic replication (a loss of normal regulatory control) of various types of cells found in the human body. Initial treatment of the disease is often surgery, radiation treatment or the combination of these treatments, but locally recurrent and metastatic disease is frequent. Chemotherapeutic treatments for some cancers are available but these seldom induce long term regression. Hence, they are often not curative. Commonly, tumors and their metastases become refractory to chemotherapy, in an event known as the development of multidrug resistance. In many cases, tumors are inherently resistant to some classes of chemotherapeutic agents. In addition, such treatments threaten noncancerous cells, are stressful to the human body, and produce many side effects. Improved agents are therefore needed to prevent the spread of cancer cells.